1. Field of the Invention
The present invention generally relates to apparatus for producing and manipulating a charged particle beam and, more particularly, to apparatus for extracting ions from a plasma and manipulating the energy of a high-current beam of ions, so extracted.
2. Description of the Prior Art
Ion beams have been used in-semiconductor manufacture for both impurity implantation and ion beam lithography for a number of years. In lithography, ion beams can provide increased resolution relative to electron beams since polymer resists can be made much more sensitive to ion exposure and exposure with a focussed beam of ions can be done with little backscatter. The use of ion beams for impurity implantation is also a common practice although implantation with a focussed beam is not yet feasible at currents which allow a practical writing speed.
Impurity implantation by means of an ion beam is desirable for a number of reasons. Perhaps most important, the beam can be mass analyzed to provide implantation of only the desired impurity material. The ion beam current and implantation energy can also be very accurately controlled to provide extremely accurate concentrations and distributions of impurities and implantation depths. Such ion implantation processes can also be carried out at low temperatures, allowing the use of low temperature masking materials.
Such implantation processes use kinetic effects and are done at high energy to implant the ions within the body of a semiconductor material. More recently, efforts have been made to use an ion beam process for purposes which require deposition on the surface of a target material, such as for welding. As can be readily understood, such a deposition process produced from an ion beam would require the energy of the ion particles to be very much lower than the energies at which implantation is performed. Such reduced energies of the ions cause some difficulties to be encountered in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. However, in such an application, the need for high beam current is not necessary because the amount of material deposited is typically small.
The formation of monocrystalline epitaxial layers of a semiconductor material, particularly with conductivity determining impurities, is often necessary in the manufacture of various types of semiconductor devices. This process is often carried out through vapor phase deposition at very high temperatures of approximately 1100.degree. -1200.degree. C. With a few exceptions (such as p-doped and intrinsic silicon) good quality monocrystalline deposition is difficult below 1000.degree. C. This high temperature requirement for forming a monocrystalline epitaxial layer therefore has the drawback that, particularly if other doped structures have previously been formed, out-gassing effects and/or out-diffusion between regions may occur. In device design, compensation of such effects is often difficult or impossible and can also limit the minimum dimension of conductivity region in the device for a particular manufacturing yield since impurity out-diffusion distances can easily dominate an epitaxial layer which is thinner than about 2 microns or a region of similar lateral dimension. Such out-diffusion due to the high temperature process also results in dopant distribution being less than optimally controllable, even when ion implantation is subsequently used to add impurities to the monocrystalline epitaxial layer. .
It should also be noted that ion implantation, by itself, does not completely eliminate the need for a high temperature process even though ion implantation can be carried out at low temperatures since the ion implantation process causes damage to the crystal lattice structure and annealing is often necessary to repair the damage before further processing can be carried out. Therefore, ion implantation does have the advantages of extremely high purity of implantation material and allows the use of low temperature masking materials and at least minimization of high temperature diffusion effects, although the diffusion effects can be limited only to the extent that annealing may be limited.
The use of an ion beam to provide a low temperature process for producing a monocrystalline epitaxial layer has been achieved and is disclosed in detail in U.S. Pat. Nos. 4,151,420 and 4,179,312 to Keller et al, assigned to the assignee of the present invention and hereby fully incorporated by reference. These techniques are characterized by the use of multi-aperture sources to obtain high ion beam current. Such multi-aperture sources produce a broad beam and it can be readily understood that a significant amount of ion beam current is lost at the mass analysis aperture if good separation of ion masses is to be obtained, even though condensing lenses may be used for each of the superimposed beams.
These techniques achieved a relatively high beam current at the target at reasonably low energies of about 500 eV. However, these currents were spread over a relatively large area of the target (e.g. a beam diameter of about 15 cm). Thus, a beam current of about 1 ma/cm.sup.2 resulted in a rate of material deposition which limited the throughput of the process. Also, by using such a large beam diameter, the epitaxial growth process was limited to performance of the process over the entire wafer and selective epitaxial growth could not even be limited to the actual chip areas, wasting beam current directed to areas of the wafer between chips.
It has also been found, by the inventors herein, that even lower ion energies are desirable for epitaxial growth during the manufacture of a semiconductor device or other object, such as a mask or calibration grid. For instance, implantation may be performed at a typical energy of approximately 20 Kev, whereas, it has been found, by the inventors herein, that energies of 2 Kev or less are required for epitaxial growth and even lower values are desirable. While the arrangements of the above-incorporated patents achieved energies of about 0.5 KeV at the target, even faster epitaxial growth for a given beam current can be achieved at energies of 50-300 eV. It has also been found that, for several reasons discussed in more detail below, energies of about 5 KeV are desirable for good performance of mass analysis where epitaxial growth consists of a material which may contain a plurality of elements (e.g. silicon and an impurity element such as boron or arsenic, depending on the conductivity type desired) and which must be deposited simultaneously at coincident locations to assure homogeneity in the epitaxial growth. It is also necessary to maintain good beam convergence and uniformity of ion current density across the beam to assure homogeneity of the epitaxial growth. Therefore, it is seen that optimal energies for mass analysis is very much higher than optimal energies for ion deposition.
As is well understood, the current of the ion beam will be largely dependent on the extraction voltage. The extraction voltage is the voltage applied to a grid-like structure to remove ions from a plasma and to form a beam. However, this voltage also initially controls ion energies due to the acceleration of the particles which occurs during extraction. The energy of the particles, to a large extent, also controls the qualities of the beam since, at higher energies, the beam scattering due to mutual repulsion between like-charged ions will have a proportionately smaller effect.
While good beam qualities have been obtained at low extraction voltages, low voltage on the source aperture also causes a problem since the acceleration of the ions is sufficiently small that the grid itself is sputtered with the material of the plasma. This can erode the grid and introduce contaminants into the process as well as greatly reducing the useful lifetime of the grid.
In summary, in the prior art, a trade-off has existed between high beam current and beam quality as well as between ion energies and beam quality. In addition, it is difficult to achieve a desirable level of beam current and beam quality consistent with durability and good service life of the extraction grid. Therefore, the prior art does not provide an arrangement which will produce a high-current ion beam with good convergence properties which can be efficiently mass analyzed and still yield a very low particle energy at the target for efficient material deposition.